There is a requirement to integrate solar panels into the structure of buildings. One concept for doing this involves installing the solar panels in place of windows. If conventional building windows are to be replaced with glass based solar panels or if flexible solar panels are to be applied to existing building window panes then it is essential that they have some partial degree of transparency and that they allow a continuous view to be obtained. A transparency in the range 5 to 20% is generally regarded as necessary. This is presently achieved in several ways.
In one case opaque square crystalline silicon or poly-silicon solar cells are used that are separated from each other in one or two directions in order to allow light to pass though the gaps. Since the cells generally have dimensions between 100 and 150 mm and electrical connections need to be made between cells this method leads to a complex window structure that does not permit a continuous view to be obtained.
In another case lasers have been used to cut apertures through silicon solar cells to create a degree of transparency. Sunways A G of Konstanz, Germany for example cut 64 square apertures each of about 5×5 mm in 125×125 mm solar cells giving an optical transparency of about 10%. Such a perforation method is slow and costly and in addition does not permit a continuous view.
In another case large opaque thin film solar panels are made to be partially optically transmitting by laser scribing through the opaque layers in a similar way to that used for creating the series interconnections between the multiple cells. In order to obtain the required optical transparency which is usually in the range 5 to 20% multiple parallel laser scribes are made along the panel in the direction perpendicular to the cell interconnection scribes. In order to carry out such a process in an economically viable time, i.e. comparable to the time taken to make the interconnect scribes, and to allow the required optical transmission it is necessary to make scribes that are much wider than the interconnect scribes. Such wide scribes are readily visible and do not allow a continuous view. U.S. Pat. No. 6,858,461 describes a process in which the scribe lines are in the direction perpendicular to the interconnect scribes. The lines may also be made on a graded pitch in order to vary the optical transparency in one dimension.
None of the processes described above readily allows partially transmitting solar panels to be made that allow a continuous view. For a continuous view it is necessary that the optically transmitting structures are indiscernible so that the whole panel appears to be partially transparent. One way of achieving this is to remove the opaque coating in the form of an array of small areas that are too small to be seen by eye at a reasonable distance from the panel. U.S. Pat. No. 4,795,500 describes the use of regular arrays of circular, triangular, square, hexagonal and polygonal shaped holes through the opaque layers on a solar panel. In that invention the holes are formed by selective chemical etching of the opaque layers following a photo-lithographic process involving application of a resist layer, which is exposed and developed to form a contact mask. This type of process is slow and costly and because a mask is required to define the hole pattern change of hole shape, pitch and uniformity is difficult.
Hence it is seen that in order to produce partially transparent solar panels that can be readily integrated into buildings in the form of windows, awnings and roof lights and fully satisfy aesthetic requirements in terms of appearance and allowing the presentation of 2D images a new type of high speed and low cost process is required. Such a process is revealed here.
US 2007/0007627 A1 discusses the use of an ink jet head to apply droplets of etchant fluid to a thin film crystalline silicon solar panel for the purpose of forming contacts to n+ and p+ regions on opposite sides of a silicon layer. Each droplet of the etchant fluid forms an aperture in an organic resist layer applied to the top of the panel active silicon layer. Subsequently these apertures are used as masks in further etching processes to create paths which extend partway through the silicon layer as far as the lower doped region. A metal layer is then applied into these paths to form electrical contacts with the lower doped region.
Reference herein to an ink-jet print head is to be understood to refer to a print head with one or more nozzles from which droplets of fluid can be ejected at predetermined times, e.g. under the control of a computer program. The type of print head used in an ink-jet printer is suitable for this. Such print heads are well known so will not be described further.
The present invention seeks to overcome limitations of the prior art and to provide solar panels which are partially transparent and have far greater opportunities for providing aesthetic designs and provides a new method and apparatus for forming such panels.